It is often necessary as for example in air conditioning and heating systems to eliminate or prevent the formation of bubbles such as gas bubbles in fluids flowing through pipes and other conduits. Accordingly visual displays such as signt glasses are employed to enable fluid flow to be monitored. However, depending upon the type of fluid and type of conduit, such displays cannot be or are not always used. Moreover, even when such displays are used, periodic visual inspection by trained observers is necessary, and, even under these conditions, dangerous bubble conditions can develop between inspection periods and thus be undetected. The present invention is directed toward electronic apparatus which will automatically and continuously detect the presence or absence of bubbles in fluid flows and which will produce an electrical signal which can be used to sound an alarm, operate control devices or initiate other immediate corrective action. Since the apparatus, once installed, can be operated remotely, it can be used in radioactive and other environments in which operator access is dangerous or forbidden.